As is known, a radio receiver, in particular in a multimedia system of a motor vehicle, is able to receive a radio signal, in particular an FM radio signal, FM being the acronym of “frequency modulation”.
Such an FM radio signal, received in modulated form by a radio receiver, is subjected to various sensors and to suitable filtering so that the corresponding demodulated radio signal is able to be played back under good conditions, in particular in the passenger compartment of a motor vehicle.
Those skilled in the art know the operating principle of an FM, that is to say frequency-modulated, radio signal received by a suitable radio receiver, with a view to being demodulated and then played to listeners.
A known problem that relates to the reception of an FM radio signal via a mobile radio receiver, in particular one incorporated into a motor vehicle, resides in the fact that the FM radio signal emitted by an emitter may be reflected by natural obstacles or buildings for example, before being received by an antenna of the radio receiver. In other words, the emitted radio signal, before being received by an antenna of the receiver, may have followed various paths, of relatively long or short length. The emitted signal may furthermore, because of masking, not be received at all by the antenna of the radio receiver.
As a result thereof a selectivity is necessary, because a given radio signal may be received by one antenna several times, with various time shifts. This problem is known to those skilled in the art, who generally refer to it as “multi-path”.
In addition, to mitigate the aforementioned drawbacks relative to multi-path and masking, it is known to equip radio receivers with at least two separate antennas that are said to create “phase diversity”.
Phase-diversity systems comprising two antennas are one known solution to the problem of generating frequency selectivity with a view to processing interference due to multi-path in motor-vehicle radio receivers.
The principle consists in combining the FM radio signals received by two separate antennas of a radio receiver, in order to make, virtually, the assembly formed by said two antennas directional, in order to privilege a desired radio signal reaching the antenna array at a certain angle, to the detriment of an undesired radio signal reaching the antenna network at a different angle.
To mitigate the effect of the spatial and temporal interference induced by the multi-path effect, systems for achieving channel equalization by means of a specific configuration of an impulse response filter (also referred to as an “FIR”) exist, in order to equilibrate the transfer function of the channel.
In this prior art, multi-tuner receivers thus employ two types of processing, which are carried out separately, the spatial processing with “phase diversity” being carried out upstream of the temporal equalization of the channel.
Furthermore, in the prior art, algorithms for removing multi-path signals are generally of the “constant modulus” type. Specifically, the principle of frequency modulation ensures that the emitted radio signal has a constant modulus. Thus, computational algorithms called constant modulus algorithms (CMAs) have been developed and those skilled in the art are constantly seeking to improve them, with for main constraint to ensure, after computation, a substantially constant modulus of the radio signal combined within the receiver, after processing.
CMA algorithms are iterative computational algorithms the objective of which is to determine the real and imaginary parts of complex weights to be applied to the FM radio signals received by one or more antennas of a radio receiver, with a view to combining them, so as to remove from the combined radio signal the interference due to multi-path.
It is therefore a question, in the prior art, of determining the components of a spatial filtering, by means of a first implementation of a CMA algorithm, then the components of an impulse response filter, for the temporal filtering, by means of the implementation of a second CMA algorithm.
FIG. 1 shows a schematic representative of the prior art, in which two antennas A1, A2 respectively receive radio signals X1,n, X2,n corresponding to an emitted FM radio signal, via respective input stages FE1, FE2. Two successive filtering stages are implemented to achieve the recombined signal Yn intended to be played. Firstly, there is a spatial filtering stage G1 and G2, respectively, then a temporal filtering stage W.
With reference to FIG. 1, a first set of equations of a system with “spatial diversity” results there from:zn=G1,nS,X1,n+G2,nX2,n JCMA=E{(|zn|−R)2}
where G1,nS, G2,nS are scalars of complex weights, for the spatial filtering of the signals X1,n, X2,n received by each of the antennas A1, A2; JCMA is the cost function to be minimized by means of a CMA algorithm and R is a constant to be determined, corresponding to the constant modulus of the combined signal.
A second set of equations of a system with “temporal diversity” results there from:yn=(Wnt)TZn JCMA,=E{(|yn|−R)2}
where Wnt is a matrix of complex weights the components of which correspond to the coefficients of an impulse response filter to be applied to the signal Zn for the temporal filtering, Zn being composed of successive samples of the signal zn issued from the spatial filtering stage; JCMA, is the cost function to be minimized by means of a CMA algorithm and R is a constant to be determined, corresponding to the constant modulus of the combined signal.
However, as the spatial filtering is performed upstream and independently, i.e. without taking into account the time dimension of the interference, problems arise. Specifically, a first iterative CMA algorithm is implemented for the spatial filtering. The fact that the time issue is not taken into account at this stage means that the implemented algorithm may at any moment hop to an adjacent radio signal. The temporal filtering performed subsequently may then have substantial difficulty converging, or even not converge.
The high number of unknowns and the absence of correlation between these unknowns makes rapid determination of stable solutions particularly difficult.
As is known to those skilled in the art, this difficulty with rapidly converging to correct and stable solutions is particularly present in the field of FM radio reception, because the only certain constraint exploitable a priori by algorithms resides in the fact that the modulus of the envelope of the frequency-modulated radio signal remains constant.
However, on the other hand, the antennas A1, A2 each receive a plurality of radio signals, corresponding to the emitted radio signal having followed various paths, which are either direct or with one or more reflections, and a complex weight must be determined with a view to being applied to each of these radio signals. The equation contains a high number of unknowns and the objective of the CMA algorithms is therefore to determine the best solutions, among a set of non-optimal solutions allowing a constant modulus of the combined radio signal to be ensured.
More particularly, in scenarios where the desired radio signals coexist with radio signals transmitted over adjacent frequency channels, this problem of convergence is more pronounced. It often occurs that the complex weights obtained with CMA algorithms privilege adjacent radio signals to the detriment of the desired radio signals. Stability problems are thus particularly frequent.